In accordance with a rapid increase in the use of renewable energy, the necessity for an energy storage device using a battery has rapidly increased. Among these batteries, a lead battery, a nickel/hydrogen battery, a vanadium battery, and a lithium battery may be used. However, since the lead battery and the nickel/hydrogen battery have significantly low energy density, they require a large space in order to store the same capacitance of energy therein. Further, in the case of the vanadium battery, the vanadium battery uses a solution containing a heavy metal which causes environmental contamination, and a small amount of materials may move between an anode and a cathode through a membrane separating the anode and the cathode from each other, which deteriorates performance, such that the vanadium battery cannot be commercialized on a large scale. The lithium battery having significantly excellent energy density and output characteristics is significantly advantageous in view of a technology. However, the lithium battery is disadvantageous in view of economic efficiency when being used as a secondary battery for large scale power storage due to the scarcity of a lithium source.
In order to solve this problem, many attempts to use a sodium resource sufficiently present on Earth, have been conducted. Among them, as disclosed in US Patent Laid-open Publication No. 20030054255, a sodium-sulfur battery having a form in which a beta alumina having selective conductivity for a sodium ion is used, an anode contains sodium, and a cathode contains sulfur has been currently used as a large scale power storage device.
However, in the existing Na based secondary battery such as the sodium-sulfur battery or a sodium-nickel chloride battery, conductivity thereof and melting points of battery compositions should be considered. For example, the sodium-nickel chloride battery has an operation temperature of at least 250° C. or more, and the sodium-sulfur battery has an operation temperature of at least 300° C. or more. Due to this problem, there are many disadvantages in terms of economical efficiency of manufacturing or operating the Na based secondary battery while maintaining a temperature and sealability of the battery and reinforcing the safety thereof. In order to solve the above-mentioned problem, a room-temperature Na based battery has been developed, but the output thereof is significantly low, such that the room-temperature Na based battery has significantly low competitiveness as compared with the nickel-hydrogen battery or the lithium battery.